In the past, a GE-PON (Gigabit Ethernet (registered trademark) Passive Optical Network) system described in IEEE (Institute of Electrical and Electronics Engineers, Inc.) 802.3ah™ splits one optical fiber with a splitter, which is a passive element, and accommodates a plurality of ONUs (Optical Network Units) to thereby economically provide FTTH (Fiber To The Home) and FTTB (Fiber To The Building).
On the other hand, in the GE-PON system, it is likely that communication disconnection occurs in all the ONUs accommodated in the same splitter when a failure of a trunk line fiber occurs. There is also a problem in that a large number of service subscribers are affected. It is likely that this problem is allowed in the FTTH for which personal use is mainly performed at a relatively low service charge. However, in the FTTB that is a service for business, it is likely that data related to a mission critical task is transmitted and received. Therefore, it is requested that the communication disconnection does not occur even when a failure occurs. Therefore, an examination concerning PON protection for making a system redundant and switching the system when communication disconnection due to a failure occurs is performed.
As a PON protection system for B-PON (Broadband Passive Optical Network)/G-PON (Gigabit Passive Optical Network), in Non-Patent Literature 1 described below, a system (Type B) for duplexing a PON terminal end on an OLT (Optical Line Terminal) side and a trunk line fiber, and a system (Type C) for making a PON section completely redundant are recommended.
In Type B of the systems, optical interfaces are reduced to one by using a 2:N splitter and a section between the 2:N splitter and the OLT is made redundant. Type B is effective for failures such as a failure of an OLT interface and a trunk line fiber disconnection. In Type C, a redundant ONU including two 1:N splitters and two optical interfaces is necessary. Because the PON section is made completely redundant, Type C is effective for more kinds of failures such as a failure of the OLT interface, trunk line/branch line fiber disconnection, and an ONU interface failure.
A method of performing redundancy using an optical switch, always operating a currently-used system and a standby system, and reducing a switching time due to a failure is also proposed (see, for example, Patent Document 1 described below).    Patent Document 1: Japanese Patent Application Laid-open No. H9-284325    Non-Patent Literature 1: ITU-T, “A broadband optical access system with enhanced survivability”, Recommendation G.983.5, 2002